Some scanning systems which determine dimensional and topographical information from a clothed individual use ultra-wideband electromagnetic radiation. These systems can non-intrusively determine the physical measurements of an individual and render a display image. U.S. Pat. No. 6,507,309 entitled Interrogation of an Object for Dimensional and Topographical Information discloses a method of determining the dimensions of a human body using electromagnetic radiation in the frequency range of about 200 Megahertz (MHz) to about 1 Terahertz (THz). In a most preferred form, this reference discloses a range of about 5 Gigahertz (GHz) to about 110 GHz.
Scanning systems such as this transmit signals to a human target only a few feet away from a an array of transmitting and receiving elements. Accordingly, signal levels are very low to avoid unhealthy radiation exposure. An ultra-wide frequency scan is desirable to obtain target topography definition that is unavailable at lower bandwidths. Operation of such systems is also designed to be swift such that the human target does not have to remain motionless for an extended period of time. Thus, scan times generally need to be under a minute in length for comfort of the human target. Accordingly, a continuous and rapid sweep of the ultra-wide bandwidth frequency set is needed as the array of transmitting and receiving elements are moved around the individual being scanned. Such ultra-wideband scanning transceivers generally exhibit a non-uniform gain in the frequency response characteristics across the frequency band of interest.
In typical communication systems, narrow band transmission techniques are used which can avoid excessive frequency response non-uniformity. For example, in narrow band systems requiring only a few MHz in bandwidth, it is relatively easy to tune the high frequency components to achieve optimum linearity performance. There, the required bandwidth allows operation across a frequency range where the frequency response of the transmitter or receiver components are more closely related and component tolerances are controllable. However in an ultra-wideband system (e.g. 5 GHz or more) tuning the frequency response of the system to be optimal is a much more challenging task because of the inherent variation of the system components over the ultra-wide frequency range. All components show some amount of gain variation when used over a wide bandwidth. But, in the ultra-wide bandwidth systems, the gain variation in the components over the frequency range can be severe. For example, the semiconductor components, cabling and antennas that are used in the total system can have gain variation in excess of 20 dB. This variation reflects directly on the signal to noise quality of the system. Not only is the gain variation substantial, it can occur in a relatively small region of the overall bandwidth of the system. For example, component and connection gain variations as much as 20 dB can easily be encountered in measured bandwidths as small as 0.2 GHz.
A system of cascaded narrow band transceivers multiplexed into the transceiver elements could be used to address the ultra-wideband gain uniformity issues addressed above. But, the solution is undesirable in terms of cost and complexity in transceiver equipment and the difficulty in mapping different bandwidth variations into useable acquisition results for post-processing of data. Accordingly, a gain compensation mechanism is desirable to allow the use of a single transceiver to system to sweep an ultra-wideband system across its full frequency range while maintaining a reasonable and predictable transmit and receive signal level across the entire spectrum of interest.